1. Field
Exemplary embodiments of the present disclosure relate to methods, systems, and apparatus to control ammonia slip in a selective catalytic reduction (SCR) system.
2. Discussion of the Background
Selective Catalytic Reduction (SCR) technology is one of the most common NOx reduction technologies in use today for internal combustion engines such as on-road heavy-duty as well as light-duty diesel engine applications around the world, for example, in the U.S. and Europe. A primary reagent used in SCR systems is urea. One form of urea used in SCR systems is referred to as Diesel Exhaust Fluid (DEF) in the U.S. and AU32 or ADBLUE™ in Europe, both of which are a 32.5% aqueous solution of urea in water. The injected urea decomposes in the presence of heat and moisture to form ammonia and carbon dioxide. The ammonia then reacts with NOx in the presence of the SCR catalyst to reduce NOx to nitrogen and water. The optimal operation of the SCR system may depend on an efficient control strategy which helps to minimize the overall NOx emissions with minimal ammonia slip Ammonia slip refers to the passage of unreacted ammonia emissions out of the SCR system. An ammonia oxidation (AMOX) catalyst downstream of the SCR catalyst may be needed to convert most of the ammonia slipping past the SCR catalyst to N2 and other less harmful compounds to avoid ammonia emissions from the tailpipe.
Ammonia slip is an unwanted by-product of the SCR catalytic reaction and is desired to be kept at a minimum Ammonia is a regulated emission in Europe; the average ammonia slip over the certification cycle is required to be less than 10 ppm for on-road and less than 25 ppm for non-road, and it is expected to be regulated in the US as well in the near future. Most SCR catalysts adsorb ammonia at low temperature conditions (less than about 300° C.) which may then be used for NOx reduction in the presence of the SCR catalyst. Ammonia can be desorbed when the SCR catalyst substrate is exposed to high temperature (greater than about 350 to about 400° C.) and this may result in momentary ammonia spikes. The momentary ammonia spikes not only result in increased ammonia slip but also result in unused (wasted) ammonia. It is therefore desired to have methods to avoid such ammonia slip both from a regulatory perspective and also from an SCR system operation efficiency standpoint.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.